The disclosed invention relates in general to electron beam lithography and more particularly to errors in such lithography introduced by backscattered electrons. In electron beam lithography, a substrate which is to be patterned is first covered by a thin layer of resist which is sensitive to bombardment of electrons supplied in an electron beam incident on the resist. These electrons decelerate as they pass through the resist thereby depositing energy in the resist along the trajectories of each of these electrons. In its forward path through the resist, energy is deposited in a cylinder of diameter somewhat greater than the diameter of the electron beam. The energy deposited in this way is known as the "forward scattered" energy and has a distribution as a function of the lateral displacement from the center of the beam of the form shown by curve 11 in FIG. 1.
In general the electrons in the electron beam must pass entirely through the resist layer and on into the substrate so that the entire thickness of the resist layer is exposed by the electron beam. In addition, if the initial energy of the incident electrons were such that the electrons come to rest in the resist, then charge would build up in those localized regions where the beam has been incident on the resist. Because localized regions of charge in the resist will deflect the electron beam, portions of the circuit pattern which have already been written by the electron beam would affect the portions of the circuit pattern which are subsequently drawn. Such deflection would result in errors in the pattern which is to be etched into the substrate. Therefore, the energy of the incident electron beam is typically chosen to be sufficient to insure that the electrons pass through the thin resist layer into the underlying substrate. Because the conductivity of the substrate is typically much higher than that of the resist, a localized charge build-up in the substrate will not result. In addition, the charge can be removed from the substrate by grounding the substrate. Typically, an energy on the order of 20 keV is sufficient to insure that the electrons pass through the thin resist layer as well as any non-conductive layer between the resist and substrate.
Unfortunately, the incident electrons are scattered by the atoms in the substrate so that a significant fraction of the incident electrons are scattered back into the resist film producing an undesirable exposure of the resist layer. The effect of the scattering by the substrate atoms is illustrated in FIG. 2 in which the calculated trajectories (utilizing Monte Carlo simulations presented in the article by D. Kyser and K. Murata in the article entitled "Monte Carlo Simulation of Electron Beam Scattering and Energy Loss in Thin Films on Thick Substrates" and published in Proceedings of the Sixth International Conference on Electron and Ion Beam Science and Technology Electrochemical Society, 1974, pp 205-223) are presented of one hundred 20 keV electrons incident on a 0.4 micron layer of PMMA resist on top of a silicon substrate. The energy deposited in the resist by the backscattered electrons is known as the "backscattered energy" and has a distribution as a function of the lateral displacement from the center of the beam of the form shown by curve 12 in FIG. 1. The total energy deposited by both forward and backscattered energy is shown as curve 13. The extra exposure of the resist by these backscattered electrons is known as the proximity effect and can significantly affect resolution of circuit features on the order of 1 micron or less. For an incident beam having a width on the order of 0.5 microns, the radius of the region exposed by the backscattered electrons is on the order of 3 microns. The ratio of the backscattered energy to the forward scattered energy is represented by nE. Typically, nE lies in the range from 0.7 to 0.9 for a single layer of resist on the order of 1 micron thick.
The effect of the backscattered electrons on resolution is illustrated in FIGS. 3A-3C which correspond to a situation in which a set of seven parallel lines A-G of equal linewidth and spacing are drawn on the resist by the electrom beam. The curves in FIGS. 3A-3C represent the distribution of energy deposited in the resist as a function of the lateral displacement along the surface of the resist in a direction perpendicular to the direction of the lines. Curves 31A-31G represent the forward scattered energy corresponding to lines A-G respectively, curves 32A-32G represent the backscattered energy corresponding to each of lines A-G respectively, curve 33 represents the total backscattered energy deposited in the resist by all of the backscattered electrons, and curve 34 represents the total energy deposited in the resist by both the forward scattered and backscattered electrons. It should be noted that the forward scattered energy distributions corresponding to each of the lines do not significantly overlap but the backscattered energy distributions corresponding to each of the lines do significantly overlap and result in a greater total backscattered energy in regions surrounded by other lines than for isolated lines or in regions near the edge of a group of lines.
The effect of the backscattered energy on resolution can be seen by examination of FIG. 3C. The curve in that figure represents the total energy (ie. both backscattered and forward scattered energy) deposited in the resist as a function of the lateral displacement. It is the total energy distribution which governs the way the resist dissolves during development of the resist. As is shown in that figure, because the backscattered energy is not uniform the heights of the peaks in the lines A-G are not equal. Similarly, the amount of exposure in the spaces between adjacent lines is also not uniform. Because the peak corresponding to line D is higher than the peak corresponding to line A, upon development of the resist, line D will be wider than line A. Similarly, the resist remaining in the space between line C and D after development of the resist will be thinner than the resist remaining in the space between lines A and B. The exposure by the backscattered electrons therefore produces a variable resolution which is a function of the pattern being drawn.
Three methods which have been proposed to compensate for the proximity effect are: compensation by dose correction, compensation by shape correction and compensation by the use of multi-level resist films. In compensation by dose correction, lines A and G would receive the highest dose of electrons, lines B and F would receive the next highest dose, lines C and E would receive the next highest dose and line D would receive the smallest dose. The dose selected for each of the lines could be chosen to equalize the heights of the peaks corresponding to lines A-G (see FIG. 4A). Alternatively, these doses could be selected to equalize the heights of the valleys between each of these peaks (see FIG. 4B) or could be selected to minimize some other parameter representing the effect of the backscattered energy. The dose can be controlled by regulating the beam current or by regulating the writing speed of the beam. In compensation by shape correction, the pattern which is actually drawn by the electron beam is altered slightly from the ideal pattern which is to result. This pattern is altered to make the total energy exposure have constant magnitude along the boundary of the ideal circuit pattern so that when the resist is developed a near ideal pattern will result. (See Carole I. Youngman and Norman D. Wittels, "Proximity Effect Correction in Vector-Scan Electron Beam Lithography" SPIE, Vol. 35 Developments in Semiconductor Microlithography III (1978), p. 54.). In compensation by the use of a multi-level resist, an electron absorbing layer is interposed between the resist layer and the substrate to reduce the amount of backscattered energy deposited in the resist layer.
Unfortunately, each of the compensation techniques has some serious disadvantages. A first disadvantage is that each of them only incompletely compensates for the backscattered energy distribution. In the case of multilevel resist films, there is some backscatter from the interposed absorbing layer and only partial reduction of the backscattered energy from the substrate. Additional processing steps must also be included to transfer the pattern produced in the thin top resist layer through the interposed absorbing layer to the substrate. As a consequence, the cost of the integrated circuit fabrication process is raised and its yield is lowered. The steps of transferring the pattern to the substrate can also produce undesired broadening or narrowing of features thereby adversely affecting resolution.
In the other two compensation techniques the corrections have to be computed for the particular pattern in question. That computation is typically a prohibitively lengthy and expensive process. In order to make the computation tractable, the pattern data must be partitioned. This is a difficult procedure and can cause unsatisfactory proximity effect corrections to be calculated in regions of the pattern which are near to the partition boundaries. In addition, the choice of the partition size can be ambiguous and can lead to errors. In order to achieve satisfactory compensation, the number of sections in the partition can be quite large thereby requiring a significant increase in the amount of data that must be stored to generate a pattern. The optimum correction is also a function of the resist and developer parameters thereby further complicating the computations. It has proved difficult to devise a computation technique which does not involve operator intervention, especially for patterns other than very simple ones. In the case of dose compensation, it must be possible to vary either the beam current or the writing speed of the electron beam lithography machine as the pattern is being written. This makes the design of the machine more complex than would otherwise be required. A compensation technique is therefore needed which produces a complete or nearly complete compensation for the proximity affect without unduly increasing the cost of circuits produced utilizing this technique because of added processing or design costs.